The present invention is related to a head-up display having a combiner which reflects light rays emitted from a display device toward a driver.
Various head-up displays have been proposed to allow a driver of a vehicle to see various kinds of driving information (for example, see Patent Document 1). For example, as shown in FIG. 7, the head-up displays reflect display light rays (display image) emitted from a display device 22 installed in a lower part of a windshield 21 toward a driver by a thin plate-shaped combiner 23 (a reflection member) formed on a surface of the windshield 21. Accordingly, the driver is allowed to be able to recognize display light rays 24 (display image) which forms a virtual image overlapping a scene in front of the windshield through the windshield.
According to the head-up display, the driver can significantly reduce the number of movements of line of sight, that have been conventionally performed, which is occurred by alternately and repeatedly looking at the displays of various meters embedded in a front surface of an instrument panel and looking at scenes and traffic situations ahead through the windshield. That is, it is possible to visually confirm and recognize on the windshield the displays of the various meters together with the scenes and traffic situations ahead through the windshield, without moving the line of sight and changing the focus of eyes significantly. Therefore, it is possible to secure steering stability of the vehicle.
However, when the display image is allowed to be formed and seen as a virtual image on the outside of the windshield, the driver sees a double image in which a position deviation occurs between display images 24a and 24b due to an optical path difference of the reflected light rays which are reflected by an inner surface (front surface) and an outer surface (rear surface) of the combiner. As a result, it is not possible to clearly see displayed information such as a vehicle speed and a direction signal, and there are inconveniences such as a mistake in reading of the displayed information and taking a long time to accurately read the displayed information. FIG. 8 shows the case in which display light rays (ghost image) G1 and G2 positionally deviated with respect to normal display light rays (display image) F1 and F2 for the display of a vehicle speed and the display of a direction signal (display of a left turn signal) on a display device 12 are seen. Meanwhile, in order to prevent the occurrence of such a double image, a method is considered which includes forming an AR coating (antireflection film) on a rear surface of a combiner 23 and suppressing reflected light rays (reflected image) from the rear surface of the combiner 23 to more clearly see reflected light rays from a front surface of the combiner than the reflected light rays from the rear surface. However, this method is not practical because an antireflection film treatment is required to be performed on the combiner 23 and this leads to an increase in the total cost of the head-up display.
With respect to this, a head-up display is proposed in which a front glass having a three-layer structure which functions as a combiner is used to allow display images which are reflected by an inner surface and an outer surface of the front glass and reach an observer to overlap each other to thereby form a single image (for example, see Patent Document 2). In the head-up display, principal surfaces on both sides of the front glass are not parallel to each other in a selected area. The information display entering the non-parallel principal surface of the front glass is configured so that directions of light rays which are reflected by the inner surface and light rays which approach the glass and are reflected by the outer surface are emitted from the inner surface are determined along the same line in a range of a driver's eyes. Accordingly, it is possible to reduce the frequency of the formation of a double image which is formed when the laminated front glass is used as a combiner of the display device.
[Patent Document 1] JP-A-2003-039983
[Patent Document 2] JP-A-07-195959